In today's wireless communication systems a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A wireless communication system comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions. The radio base stations may be controlled by one or more Radio Network Controllers (RNC).
The 3rd Generation Partnership Project (3GPP) has introduced a number of enhancements to High-Speed Downlink Packet Access (HSDPA) over the course of several releases. In particular, in the period from Release 8 (or Rel-8, for short) to Rel-10, 3GPP introduced support for multi-cell downlink transmissions.
In this regard, 3GPP standardized Rel-8 to include Dual-Cell HSDPA (DC-HSDPA) operation, whereby the network may schedule simultaneous transmissions on two adjacent downlink carriers to user equipment (UE). In Rel-9, 3GPP introduced support for DC-HSDPA in combination with Multiple-Input Multiple-Output (MIMO) transmissions, as well as Dual-Band DC-HSDPA. MIMO is used to improve performance by the use of multiple antennas at both the transmitter and receiver. The former provided a peak data rate of 84 Mbps while the later extended the Rel-8 DC-HSDPA feature so that the two configured downlink carriers may be located in different frequency bands. In Rel-10, 3GPP introduced 4 Carrier HSDPA (4C-HSDPA) operation which provides peak downlink data rates of 168 Mbps. In 4C-HSDPA four configured downlink carriers may be spread across at most two frequency bands. All configured downlink carriers within a frequency band need to be adjacent in 4C-HSDPA operation.
At this time, 3GPP is specifying support for Eight Carriers (8C)-HSDPA in Rel-11. This will allow peak data rates up to 336 Mbps. As in Rel-10, the eight downlink carriers can be spread across two frequency bands and all configured carriers within a band need to be adjacent. This concerns downlink but the problem of supporting high bit rates may as well relate to uplink.
Control signaling between the radio base stations or other nodes such as RNCs may be performed over Radio Link Controlling signalling. Radio Link Control (RLC) is a protocol used in mobile communication networks to reduce the error rate over wireless channels. Through the use of forward error correction and retransmission protocols, a physical layer, comprising the transmission technology, may typically deliver packets with an error rate on the order of 1%. The Transport Control Protocol (TCP) used in most IP networks, however, requires an error rate in the order of 0.01% for reliable communications. The RLC protocol bridges the gap between the error performance of the physical layer and the requirements for reliable communication over TCP networks.
The RLC protocol is responsible for the error free, in-sequence delivery of IP packets over the wireless communication channel. RLC divides IP packets, also called RLC service data units (SDUs), into smaller units called RLC protocol data units (PDUs) for transmission over the wireless communication channel. A retransmission protocol is used to ensure delivery of each RLC PDU. If an RLC PDU is missed at the receiver, the receiver can request retransmission of the missing RLC PDU. The RLC SDU is reassembled from the received RLC PDUs at the receiver.
The RLC protocol provides a reliable radio link between the network, such as the RNC, and the user equipment. RLC Acknowledged Mode (AM) provides high reliability by providing selective retransmissions of RLC PDUs that have not been correctly received by a user equipment. The RLC PDUs are sent to the user equipment in sequence and are correspondingly numbered with an RLC Sequence Number (SN). The user equipment sends a positive or negative acknowledgement for each RLC SN, to confirm whether or not the user equipment correctly received an RLC PDU with that SN. The network retransmits those RLC SNs that are negatively acknowledged.
Because the RLC SDU's can be large, in this case exemplified by IP packets, RLC provides a mechanism for segmentation and concatenation of IP packets. Segmentation allows IP packets to be divided into multiple RLC PDUs for transmission. Concatenation enables parts of multiple IP packets to be included in a single RLC PDU. The header of the RLC PDU conventionally includes a length indicator (LI) to indicate the length of bits of each IP packet to enable reassembly of the IP packets at the receiver. Whether the Length indicator is present in the PDU is indicted by a header extension field in the header.
The RLC block is the basic transport unit on the air interface that is used between the user equipment and the network, such as the RNC via the radio base station, and is used to carry data and RLC signaling. The RLC layer processes data for High Speed Data Packet Access (HSDPA) connections e.g. in the Acknowledge Mode (AM) e.g. for non-real time services and in an Unacknowledged mode (UM) e.g. for real time services. Currently, the downlink RLC throughput is limited and cannot support the data rates associated with e.g. 8C-HSDPA or 4C-HSDPA with MIMO.